Portable telephone sets or pocket telephone sets are one form of mobile telephone sets. Telephone sets carried on a motor vehicle are also one example of such telephones. Today, the portable telephones are practiced as wireless telephone systems. The portable telephone enables a communication between two parties by means of radio communication via a central office or central telephone exchange (radio station or wireless office). Radio frequencies usable in the communication are finite and the usable frequencies are limited to certain areas (referred to as "service areas"). In other words, a certain frequency range is assigned to a particular service area. Therefore, reclamation of frequency is performed in the wireless telephone system to enlarge an actually usable range of frequencies. However, there is a certain limit and this becomes a cause of raising an expense of portable telephones as compared with wire telephones.
The portable telephone enables a wireless or radio communication in the service area so that it is convenient as long as it is used in the service area. An user of the portable telephone can communicate with a third party by a portable telephone wherever he is as long as he carries the portable telephone in the service area.
There may be a case where one wants to talk to someone by a portable telephone from an area to which he is registered as a subscriber, for instance Los Angeles, to another area to which he is not registered as a subscriber, for instance New York. In such a case, a roaming service is available in order to take advantage of a wireless telephone system in New York. Therefore, the communication from Los Angles to New York becomes possible by utilizing such a service. However, the roaming service charges an user extra money. So, if he uses the roaming service quite often, a large bill will be sent to him. Then, it might be less costly to register himself, as a subscriber, with a New York area. Therefore, the portable telephone is designed such that a plurality of subscriber numbers can be registered in the portable telephone.
The portable telephone uses a battery as a power source. Therefore, a time length one can use the portable telephone is limited. Particularly, the portable telephone consumes a large amount of energy during the communication as compared with when the portable telephone is in a standby condition. In addition, since the portable telephone is easy to carry, anyone can take the portable telephone without permission of an owner of the portable telephone. The portable telephone is provided with a timer for measuring and recording a calling period of time and it is possible to know a calling period of time for each calling as well as a total calling period of time. With this function, the owner of the portable telephone can see how much the battery has been used, how much a telephone fee will reach and who has used the portable telephone.
However, conventional portable telephones cannot provide a total calling period of time for each subscriber number if a plurality of subscribers are registered in a single portable telephone, even if the portable telephone has the above-mentioned timer. Especially, if telephone fee rates vary with the subscriber numbers in a single portable telephone (it occurs because, for example, two subscriber numbers have different fee rates), it is impossible to know how much money spent for which subscriber number, based on the total integrated or added up period of time measured by the timer.